1. Field of the Invention
The present invention relates to a Time Division Duplex (TDD) switch of a TDD wireless communication system. More particularly, the present invention relates to an apparatus for protecting a receiver when a high-power transmission signal is incorrectly introduced into the receiver due to erroneous operations such as a malfunction of the TDD switch.
2. Description of the Related Art
In a Time Division Duplex (TDD) wireless communication system, a TDD switch is generally used for mode changes between a transmission mode and a reception mode. The TDD switch operates in response to a TDD control signal of the wireless communication system.
FIG. 1 is a diagram illustrating a conventional location of a TDD switch in a TDD wireless communication system.
Referring to FIG. 1, a TDD switch 107 is connected to a Power Amplifier (PA) 103, an antenna 111, and a Low Noise Amplifier (LNA) 115.
When the wireless communication system operates in a transmission mode, a signal from transmitter 101 is amplified to a high-power signal through the PA 103 and is then radiated through the antenna 111 via a transmit port 105 and an antenna port 109. The TDD switch 107 operates in the transmission mode and thus isolates the transmitter 101 from a receiver 117. Therefore, the receiver 117 can be protected against the high-power signal of the transmitter 101.
When the wireless communication system operates in the reception mode, the power signal sent from the antenna 111 is received through the antenna port 109 and a receive port 113. The TDD switch 107 operates in the reception mode and thus enables the received power signal to be sent to the receive port 113. The received power signal has significantly low power due to attenuation and noise. Therefore, the power signal is amplified by the LNA 115 which amplifies a signal while minimizing noise. The amplified power signal is received by the receiver 117.
FIG. 2 is a diagram illustrating a conventional TDD switch.
Referring to FIG. 2, the conventional TDD switch includes an isolator 203, a circulator 205, a λ/4 transmission line 209, a pin diode 211, and so on. The λ/4 transmission line 209 and the pin diode 211 are interconnected between a receive port 213 and the circulator 205.
In the conventional TDD switch shown in FIG. 2, the λ/4 transmission line 209 and the pin diode 211 are connected in a three connection configuration. The number of connection configurations of the λ/4 transmission line 209 and the pin diode 211 may be determined through simulation or theoretical calculation. In addition, the number of connection configurations may vary depending on the extent of isolation.
In the TDD wireless communication system, a transmitter including a PA may be connected to a transmit port 201. A receiver including an LNA may be connected to the receive port 213. Furthermore, an antenna may be connected to an antenna port 207 of the TDD switch.
The isolator 203 transmits a power signal only in one direction and is located between the transmit port 201 and the circulator 205. The isolator 203 is designed to pass only the power signal transmitted from the transmit port 201. Furthermore, the isolator 203 acts as a terminator for an external power signal that is reflected and returned. For example, when the power signal is not successfully radiated from the antenna and is thus reversely introduced, the circuit of the transmit port 201 may be damaged by the reflected power signal. Therefore, the isolator 203 protects the circuit of the transmit port 201.
The circulator 205 is a 3-port circuit element for branching the power signal. A resonance plate and a magnetic substance (e.g., ferrite) are placed inside the circulator 205 having a shape in which three ports are arranged by 120 degrees. The circulator 205 incurs an approximately 0.3 dB path loss when passing the power signal in a direction from the isolator 203 to the antenna port 207. Also, the circulator 205 isolates the power signal by a specific level (about 20 dB) in another direction from the circulator 205 to the receiver port 213. For example, when the TDD control signal operates in the transmission mode, the power signal amplified by the transmitter exhibits an approximately 0.3 dB path loss while passing through the circulator 205 and is then radiated through the antenna via the antenna port 207. In the direction from the circulator 205 to the receiver port 213, the power signal is attenuated by a certain level (about 20 dB). Although the power signal is attenuated by the specific level (about 20 dB), the receive port 213 may be damaged when the attenuated signal is transmitted to the receive port 213.
The TDD control signal is used to control the transmitter and the receiver of the TDD wireless communication system. In response to the TDD control signal, the transmitter amplifies a power signal to be transmitted and then radiates the amplified power signal to the antenna. In addition, the TDD control signal is used to control a bias circuit 221 which regulates a Direct Current (DC) bias supplied to the pin diode 211. The DC bias is supplied to the pin diode 211 through a transmission line, but this does not affect wireless communication characteristics. The pin diode 211 acts as a part of the TDD switch according to the DC bias. A capacitor (not shown) is provided to block the DC bias. Although not shown, it will be assumed that the capacitor for blocking the DC bias exists throughout FIGS. 2 to 6.
According to the transmission line theory, when an output port of a transmission line is open to ground, the impedance of the input port of the transmission line is expressed as Z=−jZo cot βl. When the output port of the transmission line is shorted to ground, the impedance of the input port of the transmission line is expressed as Z=−jZo tan βl. When the output port of the transmission line is connected to a 50 ohm transmission line, the impedance of the input port of the transmission line is expressed as Z=Zo=50 ohm. Here, β=2π/λ, and l is the length of the transmission line. As known, waves have the same amplitudes at λ/4, 3λ/4, 5λ/4, 7λ/4, and so on. Hence, the λ/4 transmission line 209 may be generalized as a (λ/4)*(2m+1) transmission line [m=0,1,2,3, . . . ]. The λ/4 transmission line 209 corresponds to a (λ/4)*(2m+1) transmission line [m=0,1,2,3, . . . ], where m is 0.
The pin diode 211 and the 50 ohm transmission line (receiver) are connected in parallel to the output port of the λ/4 transmission line 209. The pin diode 211 acts as a part of the TDD switch according to the DC bias. When the impedance of the pin diode 211 becomes nearly 0 (short-circuited), the parallel impedance between the pin diode 211 and the 50 ohm transmission line becomes nearly 0 (short-circuited). On the other hand, when the impedance of the pin diode 211 becomes nearly infinite (open-circuited), the parallel impedance between the pin diode 211 and the 50 ohm transmission line becomes nearly 50 ohm. Therefore, impedance changes in the pin diode 211 according to the DC bias allow the output port of the λ/4 transmission line 209 to become substantially shorted to ground or substantially connected only to the 50 ohm transmission line.
When the pin diode 211 is substantially open to ground, the output port of the λ/4 transmission line 209 is nearly connected only to the 50 ohm transmission line. Thus, according to the above expression of Zo=50 ohm, the impedance Z of the input port of the λ/4 transmission line 209 becomes nearly 50 ohm.
When the output port of the λ/4 transmission line 209 is substantially shorted to ground, according to the above expression of Z=−jZo tan βl where β=2π/λ, and l=(λ/4)*(2m+1)[m=0,1,2,3, . . . ], the impedance Z of the input port of the λ/4 transmission line 209 becomes nearly infinite (open-circuited).
In the transmission mode, when the TDD control signal is transmitted to the bias circuit 221, the bias circuit 221 supplies a forward DC bias to the pin diode 211. The forward DC bias allows the impedance of the pin diode 211 to become nearly 0 (short-circuited). Since the output port of the λ/4 transmission line 209 is connected to the pin diode 211, the impedance of the output port of the λ/4 transmission line 209 also becomes nearly 0 (short-circuited). Thus, the output port of the λ/4 transmission line 209 becomes substantially shorted to ground. According to the characteristic of the λ/4 transmission line 209, the impedance of the input port of the λ/4 transmission line 209 (a port nearest to the circulator 205) changes to be opposite to the impedance of the output port of the λ/4 transmission line 209 and thus becomes nearly infinite (open-circuited). Hence, the receive port 213 can be protected against the power signal while the TDD control signal operates in the transmission mode.
In the reception mode, when the TDD control signal is transmitted to the bias circuit 221, the bias circuit 221 supplies a reverse DC bias to the pin diode 211. The reverse DC bias allows the impedance of the pin diode 211 to become nearly infinite (open-circuited). Since the output port of the λ/4 transmission line 209 is connected to the pin diode 211 and the 50 ohm transmission line (receiver), when the impedance of the pin diode 211 becomes nearly infinite (open-circuited), the impedance of the output port of the λ/4 transmission line 209 becomes 50 ohm, and the impedance of the input port of the λ/4 transmission line 209 also becomes 50 ohm. Therefore, a path that spans from the antenna port 207 to the receive port 213 via the circulator 205 is not affected. Accordingly, most of the power signal received through the antenna can be input to the receive port 213.
The TDD wireless communication system may operate correctly without any problem. However, when the TDD switch incorrectly operates, the TDD wireless communication system may operate in the transmission mode while the TDD switch operates in the reception mode. In this case, the power signal may not be completely isolated by the circulator 205 and thus may be introduced to the receiver, which may lead to damage in the circuit of the receiver.
Furthermore, a cable connected to the antenna port 207 may be open when the TDD switch is turned off, or a high-power signal may be reflected when a Voltage Standing Wave Ratio (VSWR) of the circuit of the transmitter increases due to impedance mismatching. In this case, most of the reflected high-power signal is introduced into the receiver, which may damage the circuit of the receiver. Impedance matching is used to reduce performance degradation caused by an impedance difference between two separate connection ports. The VSWR represents a reflection amount of the power signal transmitted to the antenna port 207.
The DC bias of the bias circuit 221 cannot be supplied to the pin diode 211 when the TDD switch is turned off. This is similar to the case where the reverse DC bias is supplied to the pin diode 211. Thus, the TDD switch operates in the reception mode.
The conventional TDD wireless communication system cannot operate correctly when the aforementioned problems occur mostly because the introduction of the power signal into the receiver may damage the circuit of the receiver. Accordingly, there is a need for a TDD switch that can protect the receiver even when the TDD wireless communication system operates incorrectly.